Hey you! You out there, staring into space and thinking about that one art supply item that you don't currently have. This is my stern face, and this is me telling you that you don't need it. You already have everything you need to create art, I guarantee it.
You have all the art supplies you need!
It's time to stop shopping and start creating
It's so easy to get lured into thinking "If I only had X, I would be able to make great art." Where X may be a new wash brush, a different brand of technical markers, or a sketchbook of a slightly different size and paper composition than the five unfilled sketchbooks that you already own.
Believe me, I speak from a position of authority. In the same way that an alcoholic probably doesn't want to take advice from someone who has never been a drinker, I would never want to take art supply advice from someone who isn't constantly being lured in by the Next New Thing. And that person is me. I myself am just coming through a brief, passionate, but short-lived relationship with Copic markers. (My ardor cooled when I learned that they are not at all light fast. Good thing I found this out before I bought more than two. OK, three.)
Furthermore, it's good to be constantly trying out new techniques and new media. It's good to push yourself in new directions and try different things. I'm not saying you should stop buying art supplies altogether. (God no.)
I'm just saying, let's be honest with ourselves. "I need to buy X first" is just a cop-out. A cheap excuse to keep you out of your sketchbook for another day. Another way to blame the world around you for your own failure to create.
Hey, I hear you. I love a good excuse, myself. But I have seen too many would-be artists fall down the rabbit hole of art supply evaluation. Should I use a Pitt Artists Brush pen, or a Crow Quill dip pen, or an actual brush dipped in ink? Should I use Rapidograph pens or Pigma Micron? Which brand of watercolor paint has the Chromium Yellow which is the yellowest?
Art is about the thing you make, not the things you make it with. Presuming a base level of supplies - even just a ballpoint pen and a sheet of printer paper - you have everything you need. Stop making excuses, stop letting yourself get distracted by technical minutia which will never be solved, and start creating!